This invention relates to an adaptive revolution measuring apparatus comprising a revolution sensor and means for comparing and evaluating the sensor signal of the revolution sensor in order to generate a digital output signal corresponding thereto.
Sensors for temporally periodic signals, e.g. revolution sensors, which can be used with, e.g. anti-lock systems, anti-slip systems etc, generally deliver a relatively small, alternating sensor signal, onto which a direct voltage or offset voltage is superimposed. This alternating sensor signal, which is affected by fluctuations, is digitized via a comparator, i.e. converted into a rectangular signal corresponding thereto and fed to a further evaluation apparatus.
Measuring apparatuses of this type are known. They generally have a toothed wheel whose revolution signal is sensed via, for example, a magnetically operating revolution sensor. The sensor signal is then amplified and fed as an input signal to a comparator, downstream of which is an evaluation circuit for generating the output signal. It is possible to operate the comparator with a permanently set, i.e. constant reference voltage. However, this requires a substantial amount of effort for adjustment and high stability in all components, and for this reason a following reference voltage is frequently fed to the comparator. This following reference voltage is generally obtained via an RC element, but is effective only above a lower cut-off frequency.
This represents a substantial disadvantage in the case of, for example, anti-locking systems, anti-slip systems or tachometer transducers. As an example, an anti-locking system is consequently no longer effective when the speed of the vehicle to which it is fitted is below a few kilometers per hour. The wheels of the vehicle can thus also lock in this speed range.
An appropriate circuit is known from U.S. Pat. No. 4,293,814. This circuit likewise has a toothed wheel whose revolution signals are detected with the aid of a magnetically operating revolution sensor and fed to a downstream amplifier and to a comparator circuit and evaluation circuit in order to generate a digital output signal. The following reference voltage, which is also provided with this circuit, is obtained via two RC elements, which each use the signal shape of the alternating sensor signal to match the reference voltage. Because of the RC elements employed, this circuit also is subject to the disadvantage of not being effective below a lower cut-off frequency.
In order to avoid this problem, it is known to detect the signal shape of the sensor signal in a digital fashion, and use it for a following reference voltage. Although in the case of such a circuit no limiting lower cut-off frequency is present, there is the disadvantage that it is necessary before the start to detect one to two signal periods in order to detect the signal level. An appropriate circuit requires a comparatively large design effort, and even possibly a processor and is therefore poorly suited for mass produced items because of its cost.